Yousif, Wang and Hu (2019). Seed Science and Technology, 47, 2, 131-144. https://doi.org/10.15258/sst.2019.47.2.02
Seed dormancy and dormancy breaking of selected Acacia species from Sub-Saharan Africa
Mulik Abbaker Ibrahim Yousif 1, 2, Yan Rong Wang1* and Xiao Wen Hu1
1 State Key Laboratory of Grassland Agro-ecosystems, College of Pastoral, Agricultural Sciences and Technology, Lanzhou University, Lanzhou 730020, China 2 Department of Forestry Sciences, Zalingei University, Zalingei, Sudan * Author for correspondence (E-mail: [email protected])
(Submitted March 2019; Accepted April 2019; Published online May 2019)
Abstract
The Acacia species are widely distributed in the Sub-Sahara and are of significant importance in terms of economic value and for restoration. Seed dormancy is common in Acacia species, and it creates difficulties in seed testing and planting. In the current study, we investigated the degree of dormancy and the effect of different pre-treatments on dormancy breaking in selected Acacia species. The degree of dormancy varied among the species, they were 81% for Acacia nilotica, 74% for A. seyal, 15% for A. mellifera and 5% for A. senegal. The best treatments were from 10 to 90 minutes for A. seyal and 60 to 90 minutes for A. nilotica. These treatments all reduced hard seeds to 0 and did not cause damage to the seeds. The lens was identified as the initial site of water intake in the seeds of A. nilotica, whereas in A. seyal, A. senegal, and A. mellifera, the hilum and lens were both identified as the sites of primary water uptake. Further, cellular features differed among the species. A. seyal and A. nilotica seeds showed a thick but compact palisade epidermal layer; A. senegal and A. mellifera seeds had a less compact, thin palisade layer.
Keywords: Acacia, dormancy, germination, lens, seed coat, sulphuric acid, water uptake
Introduction
Many species of the Fabaceae family have impermeable seed coats, causing physical dormancy. Acacia species are considered key plants in Sub-Sahara environments, with particular importance in restoration and for planting as sources of forage and wood (Belsky et al., 1989). They also have high potential in Arabic gum production, used in food, medicine and for pharmaceutical purposes, and are useful to increase soil fertility (Alamgir and Hossain, 2005; Dossa et al., 2010). Acacia seyal Delile, A. nilotica (L.) Willd. ex Delile, A. mellifera (M. Vahl) Benth. and A. senegal (L.) Willd. are among the most promising fodder trees and their flowers, bark, fruits, gum, roots and stems are used
© 2019 Yousif et al. This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: https://creativecommons.org/licenses/by-nc/4.0
131 MULIK ABBAKER IBRAHIM YOUSIF, YAN RONG WANG AND XIAO WEN HU for medicinal purposes (Alamgir and Hossain, 2005). Seeds of Acacia species exhibit a vast range of variation in colour, size and shape. Optimal temperature is the most essential trigger for Acacia seed germination, and seeds of most species require light to achieve high germination (Escobar et al., 2010). Physical dormancy in the seeds is due to a water impermeable seed or fruit coat (Baskin and Baskin, 1998). Physical dormancy is known to be present in seeds of Fabaceae species from temperate, arctic, subtropical and tropical regions (Baskin and Baskin, 2005). There are many ways to improve germination of physically dormant seeds: scarification by sulphuric acid is the most effective and widely used method to enhance Acacia seed germination (Argel and Paton, 1999; Nadjafi et al., 2006; Narbona et al., 2006). The period used for pretreatment with sulphuric acid varies from minutes to hours, but in most cases, is between 1 and 20 minutes (Ellis et al., 1985). Concentrated sulphuric acid scarification for breaking seed dormancy for different Acacia species is recommended by ISTA (2010). Alternatively, treatment with boiling water was found to improve germination of hard seeded species (Ali et al., 2012). The basic histological structure of permeable and impermeable testas is identical (Harris, 1987; Valenti et al., 1989). Therefore, impermeability is not a consequence of a particular layer(s) of cells, instead, it is a result of a particular chemical composition of the cells that are part of the seed coat of these species (Argel and Paton, 1999). The seed coat comprises four distinct sections, which vary in number of tissue layers and thickness (Agbagwa et al., 2004). The outer layer is often impregnated with extra lipids and waxes. However, the cuticle is the point of impermeability, and the thickness of the cuticle has been implicated in the degree of impermeability (Kolattukudy, 1981). Physical dormancy is connected histologically to surfaces of the seed coat, for example tightly packed epidermal palisade cells, and different kinds of chemical substances (including lignin, phenolic deposits, lipids, suberin and wax (Baskin et al., 2000; Baskin, 2003). In several species, the lens was reported to be the primary site of water entry into the seed (Serrato Valenti et al., 1995; Baskin et al., 2000; Baskin, 2003). Taylor (2005) proposed four stages in the seed softening process: (i) pre-conditioned; (ii) impermeable; (iii) slowly permeable at some point in the testa; and (iv) directly permeable at the lens. The fundamental structure of the physical dormancy breaking in seeds of legumes and the site of initial water entry are more controversial (Baskin et al., 2000; Baskin, 2003). The current study was designed to evaluate the effect of concentrated sulphuric acid and boiling water pretreatments on germination of seeds from four Acacia species and identify the site of initial water entry during imbibition.
Materials and methods
Seed collection and preparation Seeds of Acacia seyal, A. nilotica, A. senegal and A. mellifera were collected from Darfur, Western Sudan. The mean annual temperature is 20-30°C with a mean annual rainfall of 200-1200 mm. Seeds were brought to the Key Laboratory of Grassland Agro-ecosystems,
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College of Pastoral, Agricultural Sciences and Technology, Lanzhou University, China. In September 2016, healthy seeds were selected. These seeds were stored in plastic boxes at 5°C until used for evaluating different treatments the same month.
Sulphuric acid (H2SO4) treatment Seeds were soaked in concentrated sulphuric acid (98%) for 0, 1, 3, 6, 10, 20, 30, 40, 50, 60, 70, 80 and 90 minutes; the control treatment consisted of untreated seeds.
Hot water treatment To examine the infl uence of boiling water on the breaking of physical dormancy, seeds were soaked in hot water (100°C) for 0, 3, 6, 9, 20, 60 and 90 minutes; the control treatment consisted of untreated seeds.
Seed coat anatomy Changes in the seed coat features were evaluated using scanning electron microscopy (SEM). Acid scarification was achieved by immersing the seeds in concentrated sulphuric acid (98%) for 0, 10, 20, 30 and 40 minutes or in hot water for 90 minutes. The seeds were then dried at room temperature for 24 hours. The control treatment consisted of untreated seeds. Three seeds were chosen at random from each treatment and the control. All seeds were coated with gold and observed with a (JSM-6380LV (JEOL, Japan) scanning electron microscope at 20 kV. From all the examined seeds, two regions of the seed coat could be distinctly identified: (1) a hilum and (2) the lens.
Point of water entry during imbibition To assess the site of initial water entry into the seed, a total of 400 seeds of A. nilotica and A. seyal were scarified in concentrated sulphuric acid or hot water for 0, 1, 3, 20 and 60 minutes. In contrast, seeds of A. senegal and A. mellifera were blocked with Vaseline without scarifying in sulphuric acid or hot water. The experiment consisted of four groups of 100 seeds for each treatment as follows: (1) control, no blockage applied; (2) Vaseline applied to hilum area; (3) Vaseline applied to lens area; (4) Vaseline applied to the seed coat. For each sulphuric acid and hot water blockage treatment, four replicates of 25 seeds were used. Four replicates of 50 seed were placed on top of two layers of filter paper in 90 mm-diameter Petri dishes and covered with sulphuric acid and boiling water. The treated seeds were thoroughly washed in running tap water to remove all residues of sulphuric acid and then randomly arranged and incubated at 20°C for 12 hours. A seed was considered to have germinated when radical extension was at least 5 mm. Seeds were evaluated according to the ISTA Rules (ISTA, 2010), and the final germination percentage was determined at 21 days for each species. The experiments were laid out in a completely randomised design. Germination percentage (GP) as a proportion of seeds sown, hard seed reduction (HS), percentage of abnormal seedlings (AS) and dead seeds (DS), were calculated. Hard seed % = Number of seeds not imbibition / Total number of seeds × 100 Comparisons in germination performance across different seed pre-treatment methods were made using one-way ANOVA in SPSS version 19.0.
133 MULIK ABBAKER IBRAHIM YOUSIF, YAN RONG WANG AND XIAO WEN HU
Results
Hard seed and seed germination performance The degree of dormancy varied among the species: 81% for A. nilotica, 74% for A. seyal, 15% for A. mellifera and 5% for A. senegal. The sulphuric acid pre-treat ments had the greatest effect in hard seed reduction (figures 1 and 2). The percentage of hard seed decreased as the sulphuric acid pre-treatment duration increased. In A. seyal, 10-minutes of sulphu ric acid pre-treatment reduced the proportion of hard seeds from 74 to 0% while the 20-minute hot water pre-treatment reduced the proportion of hard seeds to 63% (figure 1). The 60-minute sulphuric acid pre-treatment resulted in a hard seed reduction from 81 to 4% in A. nilotica compared with hot water pre-treatment which reduced the proportion of hard seeds to 54.5%. The best treatments were from 10 to 90 minutes for A. seyal and
(A) A. seyal 100 a ac aac baa bdaaaa a b ba a d bd a bd abd ab ab cd d bd dd
80 c AS DS b HS 60 a GP a ab a a ab a a ab ab 40 b c 20 cd
Proportion of seeds (%) d 0 0 1 3 6 10 20 30 40 50 60 70 80 90 Sulphuric acid treatment duration (minutes)
(B) 100 a aaaa a a a a a 90 a
80 AS 70 DS a ab b HS 60 a a a GP 50 40 30 20 a a a a Proportion of seeds (%) 10 a a 0 0 3 6 9 20 90 Hot water treatment duration (minutes) Figure 1. Effect of (A) sulphuric acid and (B) hot water pretreatments on the proportion % of Acacia seyal seeds that germinated (GP), were hard (HS), were dead (DS) or which produced abnormal seedlings (AS).
134 SEED DORMANCY OF ACACIA SPECIES
(C) A. nilotica 100 a a a aaaaaaac c de de de a c b c c d 90 80 c AS 70 b DS 60 HS ab b ab 50 a a a a a GP 40 30 b 20 bc 10 Proportion of seeds (%) d d d d 0 0 102030405060708090 Sulphuric acid treatment duration (minutes) (D) 100 aaa a aaa a a a aaa a 90
80 AS 70 DS 60 c HS b aaa GP 50 a a 40 30 20 a Proportion of seeds (%) 10 b b b b b b 0 0 3 6 9 20 60 90 Hot water treatment duration (minutes)
Figure 2. Effect of (C) sulphuric acid and (D) hot water pretreatments on the proportion % of Acacia nilotica seeds that germinate (GP), were hard (HS), were dead (DS) or which produced abnormal seedlings (AS).
60 to 90 minutes for A. nilotica. Most of the pre-treatments did not significantly increase the number of abnormal seeds and dead seeds compared with the control (P > 0.05). The seeds of A. senegal and A. mellifera were more sensitive to pretreatments than the other species. The germination percentage improved gradually as the pretreatment period increased from 1 to 90 minutes. The highest germination results were obtained after 50 minutes pretreatment for A. seyal and 90 minutes for A. nilotica. However, A. nilotica and A. seyal responded poorly to the hot water treatments with the highest germination of 30.5 and 39.5%, respectively. A. senegal and A. mellifera showed a normal response to the boiling water treatment. In general, the germination of hot water treated seeds was much higher than that of the control.
135 MULIK ABBAKER IBRAHIM YOUSIF, YAN RONG WANG AND XIAO WEN HU
Point of water entry during imbibition seeds The control was exposed for different times by sulphuric acid without blockage. In most treatments, the percentage of imbibed seeds increased lens blockage compared to hilum and seed coat (table 1). In contrast, the percentage of imbibed seeds of A. seyal after 20 minutes of acid scarification was significantly improved by blockage of the seed coat. Furthermore, we investigated that the primary site of water entry into the A. nilotica seeds indicated the lens the site of initial water intake. However, for A. seyal, A. mellifera and A. senegal water entered the seeds through the hilum and lens.
Table 1. Percentage of imbibed seeds of four Acacia species after 14- and 21-days incubation at 20°C, when different areas of the seed coat were blocked with Vaseline (lens, hilum or seed coat) following various dormancy breaking treatments. 14 days 21 days Duration Pretreatment Seed Seed (minutes) Control Lens Hilum Control Lens Hilum coat coat Acacia seyal Sulphuric acid 1 36a 8c 23b 3d 42a 13c 28b 6d Sulphuric acid 3 45a 20c 30b 4d 61a 25c 53b 9d Sulphuric acid 20 98a 100a 100a 87b 98a 100a 100a 88b Hot water 3 23a 8c 15b 6d 29a 11c 20b 7d Hot water 20 30a 8c 18b 4d 33a 8c 29ab 4d
Acacia nilotica Sulphuric acid 20 8a 0b 8a 0b 15ab 0c 18a 0c Sulphuric acid 60 44a 18c 32b 7d 65a 17c 43b 10d Hot water 20 5.5a 0c 4b 0c 15a 0c 13ab 0c Hot water 60 3a 0c 2b 0c 12a 0c 9ab 0c
Acacia senegal 81a 34c 48b 14d 90a 46c 71b 18d Acacia mellifera 57a 16bc 40b 2d 70a 21c 52b 7d
Means within a seed sample marked by diff erent letters are signifi cantly diff erent at the 5%.
Seed coat anatomy Three structures could be observed for the examined seeds after treatment. The hilum and lens were located in the middle of the micropyle in A. mellifera and A. senegal (figures 5, 6A-E), whereas in A. seyal and A. nilotica the hilum was on the opposite side to the lens (figures 3A-F and 4A-F). In the examined control seeds, the hilum and lens remained intact with visible cracks (figures 3A, 4A, 5A and 6A). After acid scarification for 10, 20 and 30 minutes, the hilar groove appeared more full than in the controls, and seed cracks were found with eroded areas visible in the lens and hilum. After acid scarification for 40 minutes or hot water for 90 minutes, the number of cracks in the hilum and lens increased,
136 SEED DORMANCY OF ACACIA SPECIES
(A) (B) (C)
(D) (E) (F)
Figure 3. Scanning electron micrographs of the Acacia seyal seed coat following treatment with concentrated sulphuric acid for (A) 0 minutes, (B) 10 minutes, (C) 20 minutes, (D) 30 minutes and (E) 40 minutes, or in (F) hot water for 90 minutes. H = hilum; L = lens open with destroyed palisade layers; M = micropyle.
(A) (B) (C)
(D) (E) (F)
Figure 4. Scanning electron micrographs of the Acacia nilotica seed coat following treatment with concentrated sulphuric acid for (A) 0 minutes, (B) 10 minutes, (C) 20 minutes, (D) 30 minutes and (E) 40 minutes, or in (F) hot water for 90 minutes. H = hilum; L = lens open water gap with disrupted palisade layers; M = micropyle; P = palisade layer.
137 MULIK ABBAKER IBRAHIM YOUSIF, YAN RONG WANG AND XIAO WEN HU
(A) (B) (C)
(D) (E)
Figure 5. Scanning electron micrographs of the Acacia senegal seed coat following treatment with concentrated sulphuric acid for (A) 10 minutes, (B) 20 minutes, (C) 30 minutes and (D) 40 minutes. H = hilum; L = lens open without outer permeable cell layers; M = micropyle.
(A) (B) (C)
(D) (E)
Figure 6. Scanning electron micrographs of the Acacia mellifera seed coat following treatment with concentrated sulphuric acid for (A) 10 minutes, (B) 20 minutes, (C) 30 minutes and (D) 40 minutes. H = hilum; L = lens open without outer permeable cell layers; M = micropyle.
138 SEED DORMANCY OF ACACIA SPECIES most of the lens appeared destroyed, and the water gap had opened (figures 3E, 4E, 5E, F and 6E, F). The different cellular features among the species may be divided into two groups: A. nilotica and A. seyal had an impermeable seed coat (deep physical dormancy) with a compact palisade epidermal layer, and thick tissue (figure 7A, B). Therefore, A. mellifera and A. senegal had a permeable seed coat (less physical dormancy) with a small, thin palisade epidermal layer, and seed coats with more narrow sclerified parenchyma (figure 7C, D).
( ) A (B)
(C) (D)
Figure 7. Cross-sections of the seed coat of Acacia species. (A) A. nilotica, (B) A. seyal, (C) A. mellifera, (D) A. senegal. PL = palisade layer; H = hypodermis; CU = cuticle; LL = light line arrow; SP = sclerified parenchyma layers.
Discussion
A significant variation in hard seed proportion was seen among Acacia species (15-80). This study demonstrated the effectiveness of concentrated sulphuric acid pre-treatments in breaking dormancy in Acacia species. The effectiveness of the sulphu ric acid pretreatments declined with decrease in the pre-treatment period from 10 to 90 minutes. For A. nilotica, the hard seed proportion reduced from 81 to 0% with sulphuric acid treatment and for A. seyal, from 74 to 0%. These results are in agreement with other reports that sulphuric acid improved seed coat permeability of Acacia species (Harvey, 1981; Bebawi, 1985; Mohammed, 2015). In a study by Rehman et al. (1999), treatment with 98% sulphuric acid for 10 minutes was reported to be ineffective at breaking seed dormancy of Acacia salicina Lindl., whereas 30 minutes immersion of the seeds resulted in a significant increase in germination. This supports the previous findings that the exposure time to
139 MULIK ABBAKER IBRAHIM YOUSIF, YAN RONG WANG AND XIAO WEN HU sulphuric acid is critical and needs to be quantified for each species (Baskin, 2003). The effectiveness of 80% of sulphuric acid could be attributed to the successful removal of several lignified layers in the testa, which are packed tightly together and contain water- repelling compounds. Scarification using acid may also enhance germination capacity by increasing the leaching of growth inhibitors from the seed (Baskin, 1998). According to Ells (1963), exposing the seeds to concentrated sulphuric acid can also have a negative effect as it can end up damaging the seed if the acid penetrates the seed via its exposed micropyle. There was no significant difference in the effect on germination of seeds pretreated with hot water for different lengths of time. Exposure for 3, 50 and 90 minutes resulted in the highest germination of A. seyal (99%) and A. nilotica (95%). Seed treatments with hot water have been found to improve germination of hard seeded species by improving both water and O2 permeability of the testa (Munawar et al., 2015). The blockage treatments had significant effects on seed imbibition. Our results are in line with previous studies concerning the point of water entry. Imbibition of Acacia seeds was initially reported to be restricted by the small diameter of the lens (Hanna, 1984; Barbosa et al., 1999). In estimating water entry and water gap in Delonix regia (Boj. ex Hook.) Raf., dormancy was reported to be relieved in 18 and 24% of seeds stored dry and wet, respectively, due to the lifting of palisade layers in the lens region to form a circular lid-like opening water gap (Jaganathan et al., 2017). The study concluded that, based on the results of a blocking experiment, water entered only through the lens with no secondary water entry point. Geisler et al. (2017) reported that the lens was the only water entry point in Peltophorum dubium (Spreng.) Taub. seeds after treatment at 40 and 50°C, whereas in Mimosa seeds the primary point of water uptake was the lens and micropyle. The lens is identified to be the initial site of water uptake in Fabaceae seeds with physical dormancy (Baskin et al., 2000) and has been reported in many Fabaceae species, including Albizia lophantha (Willd.) Benth. (Dell, 1980), Sesbania punicea (Cav.) Benth. (Manning et al., 1987) and Schizolobium parahyba (Vell.) S.F.Blake (Souza et al., 2012). In contrast, many studies reported the entry of water at sites other than the lens. The micropyle and hilum were identified as the water entry points in seeds of Rhynchosia minima (L.) DC. (Rangaswany and Nandakumar, 1985). Hu et al. (2009) reported that the hilum of Vigna oblongifolia A. Rich was the structure that allowed water absorption. Water absorption in seeds of Cassia species was also reported to be linked to the opening of the micropyle near the lens (Bhattacharya and Saha, 1990). Thus, differences exist in results and interpretations in the literature on the primary site of water intake into seeds (Hanna, 1984; Serrato Valenti et al., 1995; Morrison et al., 1998; Baskin et al., 2000; Baskin, 2003, Ma et al., 2004; Das and Saha, 2006). Scanning electron microscopy (SEM) images of the seed coats of untreated seeds has confirmed the lens-like structures in the Fabaceae (Burrows et al., 2009; de Paula et al., 2012). Even though we strongly supported the existence of three recognisable stages and a minute opening at some points in the testa, for example, lens, hilum, and micropyle, the differences between the studied species is the site of the hilum which is sited opposite of the lens in some species. Another unmeasured factor may also be involved, such as non-
140 SEED DORMANCY OF ACACIA SPECIES uniform impermeability over the testa surface and distribution of chemical compounds in the testa layers and physiological embryo-testa interaction (Weker, 1980). According to Morrison et al. (1992), the difference between the seed size having relatively smaller seeds. However, a different mechanism may be responsible for the loss of seed dormancy during storage whereby seed coats become water permeable increasingly (Cavanagh, 1987). In Acacia species seeds were found to be morphologically similar, both externally and internally to most Fabaceae species. The seed coat of the four Acacia species revealed some differences. In conclusion, the results obtained in this experiment indicated significant differences in the germination of seeds for the exposure times and species all sulphuric acid pre- treatments, whereas no significant difference existed in the germination of seeds for whole the exposure times exception 90 minutes for hot water pre-treatments. The best treatments were from 10 to 90 minutes for A. seyal and were 60 to 90 minutes for A. nilotica. These treatments all reduced hard seeds to 0 and did not cause damage to the seeds. The lens was identified as the site of initial water entry in seeds of A. nilotica, whereas in A. seyal, A. mellifera and A. senegal, both hilum and lens were recognised as the sites of primary water uptake. The water entry sites after dormancy breaking were through the hilum, lens and a region of the seed coats for all species. Further, the different cellular features, among those species divided into two groups: one group, including A. seyal and A. nilotica, had an impermeable seed coat with a palisade epidermal layer compact and thickness tissue had been water penetrated as well as, the other group consisting A. senegal and A. mellifera, a permeable seed coat displayed a palisade layer small and thin cells and seed coats narrower sclerified parenchyma. However, macro-morphological seed features of Acacia taxa showed differences between seed colour, seed size, seed shape, and seed coat texture and the site of the hilum is located opposite to the lens in some species. The disparity observed in the seed coat structure among the studied species could be related to different regenerative responses to environmental conditions.
Acknowledgements
The authors are grateful to all Staff of Laboratory of Pastoral, Agricultural Sciences and Technology, particularly Dr. Li Ya Jie and Yu Ling for providing competent technical assistance. We are also thankful to Qibo Tao and Chen Dali for his help in data analyses. Finally, we would like to thank Prof. Carol Baskin and Prof. Jerry Baskin for their advice and valuable comments on this manuscript that have helped to enrich it. The Chinese Government Scholarship provided financial support for the research for Foreigner Students and the ‘111’ Project (B12002).
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